A British couple who witnessed the last moments in the life of Welsh backpacker Katherine Horton last night retraced their steps along a Thai beach with detectives who are investigating the 21-year-old's murder on the holiday island of Koh Samui.

Chris and Jill Burrows took 30 detectives along Lamai beach, explaining how, when they took the same route at about 9.30pm last Sunday, they saw a young woman in a black dress, believed to be Katherine, talking on her phone. But when they strolled back later, only the phone remained, lying on the sand, still lit up from recent usage.

'She came out of the dark, we were a bit startled,' said Mr Burrows, who comes from Silsoe in Bedfordshire. 'She was chatting on the phone. She was very happy.' 'She was all bubbly,' Mrs Burrows added.

Katherine, 21, a third-year psychology student at Reading University, was thought to have been on the phone to her mother. Mr and Mrs Burrows told the police officers that when they returned 10 minutes later they found a mobile phone lying at the water's edge.

They cannot remember exactly where on the beach it was. 'It was very dark that night,' Mr Burrows said. 'We saw a little light in the dark, the water was lapping against it. It was so dark we couldn't see it was a phone at first.'

Detectives believe that Katherine, from Thornhill near Cardiff, was beaten and raped before she died from suffocation. She was found the following morning in the sea, about two miles away, by a jet skier. It is still not clear exactly where she died - whether it was indeed on the beach, close to where Mr and Mrs Burrows saw her, or if she was abducted, taken away from the beach and then dumped in the sea.

Mrs Burrows said that Katherine was not the only person they saw on the beach that night. 'It was a very dark night and we saw people, but no one we would recognise now,' she said. 'They were just shadows.'

Police have taken DNA samples from 32 people, including six foreigners. 'We have checked three of the foreigners' samples and three of the Thais' samples against the sperm that we found in her, and they are all negative,' said the chief investigator, Major-General Assiwin Kwanmuang, yesterday. 'We hope to do the rest tomorrow.'

The Burrows, who had been on the island since 29 December and walked the same route every night after dinner, admitted they could not be sure exactly where they found the phone. 'It was so dark, we cannot remember the place,' Mrs Burrows said. On their walk they stopped at several places; near the beach cabin at the New Hut bungalows where Ms Horton had stayed with her friend Rachel Adams, a hundred yards further up the beach, and one other place.

Mr Burrows said they took the phone to the nearby Buddy's restaurant. 'We went to reception, said we had found this phone and handed it in,' he said. He said they only came forward yesterday because they learnt of the murder on Friday. 'We left the island two days later and it wasn't until last night that we saw in the newspaper in Bangkok there had been a murder here,' he said. 'We knew we had found the phone, and then in today's paper they were talking about a middle-aged couple handing in the phone, and we realised we could be of assistance.'

The Burrows were flown from Bangkok to Koh Samui yesterday evening. Detectives hope that the couple will remember more if they take the walk again. 'We need them to relax for a while and then we will ask them to take the walk again,' said Colonel Preecha Thimanmontri. He said that the phone's Sim card was missing.

Another senior detective, Major-General Suthan Chayanon, described the Burrows' evidence as crucial but laughed when asked if an arrest was imminent. 'We are doing our best,' he said. 'Please give us time.' He insisted that the investigating team, which comprises more than 100 officers, including four generals, is working well together. 'The cooperation is very good,' he said, adding that they had learnt from previous cases, including the still unsolved murder of another Welsh backpacker, Kirsty Jones, in the northern city of Chiang Mai in August 2000.

Katherine's father, Ian, and brother, Christopher, were due to fly back to Britain with her body yesterday, but the Foreign Office declined to comment on their whereabouts.